Selective hydrodeoxygenation of α, β-unsaturated carbonyl compounds to alkenes

Achieving selective hydrodeoxygenation of α, β-unsaturated carbonyl groups to alkenes poses a substantial challenge due to the presence of multiple functional groups. In this study, we develop a ZnNC-X catalyst (X represents the calcination temperature) that incorporates both Lewis acidic-basic sites and Zn-Nx sites to address this challenge. Among the catalyst variants, ZnNC-900 catalyst exhibits impressive selectivity for alkenes in the hydrodeoxygenation of α, β-unsaturated carbonyl compounds, achieving up to 94.8% selectivity. Through comprehensive mechanism investigations and catalyst characterization, we identify the Lewis acidic-basic sites as responsible for the selective hydrogenation of C=O bonds, while the Zn-Nx sites facilitate the subsequent selective hydrodeoxygenation step. Furthermore, ZnNC-900 catalyst displays broad applicability across a diverse range of unsaturated carbonyl compounds. These findings not only offer valuable insights into the design of effective catalysts for controlling alkene selectivity but also extend the scope of sustainable transformations in synthetic chemistry.

sites and then presenting the importance of this work.
2) In Table 2, the yields are much lower than the conversions.Please add other products and their selectivities.It is important for understanding the reaction mechanism to analyze the reaction path.
3) Reaction kinetics of selective hydrodeoxygenation of cinnamaldehyde over all the ZnNC-X catalysts should be provided to better understand the reaction process proposed by the authors.
5) The authors proposed that the Lewis acidic-basic sites were responsible for the selective hydrogenation of C=O bonds while the Zn-Nx species served as the active sites for the hydrodeoxygenation step.Are these two active sites compatible in the separate catalytic steps?Does the presence of the Zn-Nx species inhibit the selective hydrogenation of C=O bonds over the Lewis acidic-basic sites through competitive adsorption of reactants?
6) It seems that the Zn-Nx species also contributed to the selective hydrogenation of C=O bonds because ZnNC-1000 and ZnO also produced cinnamyl alcohol from cinnamaldehyde, although the yield was low.In the second step, the Lewis acidic-basic sites may also activate IPA to facilitate the hydrodeoxygenation of cinnamyl alcohol.
7) The format of the references is wrong and there are some mistakes in the manuscript.For example, in page 5 line 120, "CAL selectivity" should be "the conversion of CAL".The author is suggested to check through the paper.

Dear the editor and reviewers:
We would like to thank the editor and the reviewers for the positive and constructive comments regarding our paper.As you are concerned, there are several problems that need to be addressed.According to reviewers' useful suggestions, we have made corrections to our previous draft, the detailed corrections are listed below.Furthermore, we have resubmitted an updated manuscript, wherein the modifications are indicated in blue.
Reviewer #1 (Remarks to the Author): This is an interesting paper that reports the catalytic conversion of unsaturated carbonyl compounds to alkenes on the ZnNC-X catalysts.The authors proposed a two-step mechanism in which the carbonyl group might be hydrogenated into hydroxyl group by the surface Lewis acid/base sites without affecting the C=C bond followed by the hydrodeoxidation of the hydroxyl group to form an alkene on the Zn-Nx sites.Many characterization techniques were used to study the structures of catalysts and surface adsorption states.Generally speaking, the work was well designed and done and the paper was well organized and written.However, there were still some drawbacks that need authors' attention.

Response:
We appreciate the favorable assessment of our work.The high evaluation from the reviewer is truly encouraging.Taking into account your constructive feedback and suggestions, we have revised our manuscript, thereby enhancing the quality of our work even further.
(1) In Fig. 1 (c), the authors used decimal points for surface areas, which was not appropriate since the BET method has the theoretical error of 10%.(2) The authors did not mention the roles played by carbon which was the most abundant element in their catalysts.Surface functional groups such as -COOH, -C=O and -OH must grow on carbon surfaces which are usually the acidic and basic sites which might play the roles for the hydrogenation of carbonyl to hydroxyl group via the CTH as the authors expected.

Response
Response: Thanks for the reasonable comment.The NC-900 catalyst was prepared by pyrolysis of chitosan at 900℃ under argon atmosphere directly.This catalyst did not contain zinc element, but contained C, N, O and H elements.We first tested the functional groups contained in the catalyst We then tested the catalytic activity of NC-900 and no products were detected as shown in Table 2 (Table 2, Entry 11).These findings suggest that the carbon-containing group may not serve as the catalytically active site."Furthermore, it was established that the presence of zinc is essential for the reaction, as evidenced by the use of the NC-900 catalyst without zinc, which resulted in no product formation (Table 2, Entry 11)."(Please see Page 4 in the revised manuscript, marked in blue) "The preparation of NC-900 catalyst.The NC-900 catalyst was prepared by the pyrolysis of chitosan.Typically, chitosan was pyrolyzed at 900 ℃ for 3 h under argon atmosphere (The heating rate is 5 °C•min −1 , and the argon purge rate is 60 ml/min.).Finally, the resulting black solid obtained by calcination was ground with an agate mortar."(Please see Page 11 in the revised manuscript, marked in blue) Table 2 Selective hydrogenation of cinnamaldehyde and hydrodeoxygenation of cinnamyl alcohol over various catalysts.
(3) The authors said that in the Zn-Nx, Zn cations were the Lewis acid sites while N the Lewis base sites, which were responsible for the hydrogenation of carbonyl to hydroxyl group.At the same time, the authors also demonstrated that the second step (the hydrodeoxidation of hydroxyl group to alkenes) occurred on the Zn-Nx sites.Can the authors confirm that the two steps occurred on the same Zn-Nx sites (or distinguish them)?
Response: The Zn-N x sites also served as the Lewis acid in the initial step., resulted in a substantial decrease in CAL conversion from 95.0% to 53.7% when 20 mg KSCN was added to the reaction (Supplementary Table 4, Entry 4).In the subsequent step, both pyridine and KSCN reduced the conversion of COL, while the impact of boric acid was minimal (Supplementary Table 4, Entry 6-8).These outcomes further indicate that the active site for the initial step was a Lewis acid-base site, whereas for the second step, the active sites consisted of Zn-N x , as evidenced by the effect of KSCN on the reaction (Supplementary Table 4, Entry 12).Additionally, the Zn-N x sites also served as the Lewis acid in the initial step."(Please see Page 8 in the revised manuscript, marked in blue) "In the case of ZnNC-X (X=800-1000), as the reaction time extended, the yield of cinnamyl alcohol initially rose and then declined.Concurrently, the production of alkenes continued to increase, and the rate of alkene formation accelerated after nearly complete conversion of the CAL substrate.It could be seen from the time curves that the presence of CAL partially inhibited the hydrodeoxygenation of COL (Supplementary Fig. 7c-f).This indicated that CAL and COL competed for adsorption at the same site (Zn-N x site)."(Please see Page 10 in the SI, marked in blue).We also demonstrated the mechanism through deuterium labeling experiments, establishing that the process involved Lewis acid-mediated intermolecular hydride transfer from the β-H in isopropyl alcohol to the carbonyl group.Furthermore, infrared spectroscopy indicated that aldehydes and isopropanol were adsorbed at the same site, providing further evidence for the MPV mechanism. 38,39 (5) There were some experimental evidences provided by the authors regarding the two-step mechanism.However, the catalyst surfaces were not involved in the mechanism.Hope that the authors may provide with some schematic diagrams depicting the surface structures of their catalysts in terms of Lewis acid/base and Zn-Nx sites interacted with adsorbed species showing the micro-processes for the two steps in the mechanism.I just did not get the point how the hydrodeoxidation of hydroxyl group occurred on the Zn-Nx sites, neither the hydrogenation of carbonyl to hydroxyl group catalyzed by the Lewis acid/base sites.

Response:
We appreciate the question and apologize for not clearly explaining the mechanism that occurs on the surface of the catalyst.We drew a schematic to further clarify the mechanism (Supplementary Fig. 23).(Please see Page 21 in the SI, marked in blue).
"Based on the above experiments and in combination with relevant literature 23 ,47 ,48 , we proposed a possible mechanism (Supplementary Fig. 23).Initially, isopropyl alcohol was adsorbed onto the surface of the ZnNC catalyst, interacting with Lewis acid-base sites.The hydroxyl O of isopropyl alcohol was adsorbed at the Lewis acid (Zn-N x ) site, and the base site pyridine N lead to the dissociation of isopropyl alcohol.Subsequently, the carbonyl group of CAL adsorbed at this site and underwent the MPV reduction by forming a six-membered ring transition state.IPA was oxidized to acetone (ACE) and desorbed, and CAL was reduced to COL.Following this, another molecule of isopropyl alcohol attacked the allyl carbon on the adsorbed cinnamyl alcohol.The process possibly generated two ether intermediates through dehydration, which then underwent (6) Some English grammar errors: (i) "To further clarify the important role of Lewis acid-base site for the catalytic selective hydrogenation.The control poison experiments were performed" (lines 119-120); (ii) "Interestingly, when the catalyst was poisoned with pyridine and boric acid, which targeted the acid and base sites, respectively.There was little impact on the catalytic performance (lines 200-202)".In each case, the two parts must be one sentence separate by a comma.
Response: Thanks for your careful checks.We are sorry for our carelessness.Based on your comments, we have made the corrections.In view of above points, I suggest that a revision is necessary before the paper is considered for publication in Nature Communications.
Reviewer #2 (Remarks to the Author): This paper describes an interesting catalytic process for producing alkenes from α, β-unsaturated carbonyl compounds via selective hydrodeoxygenation over a bifunctional catalyst.The authors carefully demonstrated that the Lewis acidicbasic sites were responsible for the selective hydrogenation of C=O bonds while the Zn-Nx species served as the active sites for the hydrodeoxygenation step.Because of this bifunctional mechanism, the ZnNC-900 catalyst showed a high catalytic activity in this challenging selective hydrodeoxygenation reaction.This work provided a new pathway for the production of olefins from biomass, which is significant for the development of green catalytic processes.Also, the bifunctional catalytic mechanism provided insights for designing efficient cascade catalysts.Therefore, this paper can be published in Nature Communications after addressing the following concerns.
Response: We are very grateful for your insightful and constructive comments, which we find very helpful in revising our manuscript.1) Are there other catalysts reported previously for the transformation of α, β-unsaturated carbonyl compounds into alkenes via selective hydrodeoxygenation?The Introduction need to be updated for highlighting the advantages of the ZnNC-X catalyst containing two different active sites and then presenting the importance of this work.
Response: This is a helpful suggestion that reminds us to describe a more comprehensive research background.There are some relevant reports on the transformation of α, β-unsaturated carbonyl compounds into alkenes via selective hydrodeoxygenation.These references were cited in the text.We have added a description in the introduction, highlighting the advantages of the ZnNC-X catalyst containing two different active sites, and then presenting the importance of this work.Here is what was added in the introduction.
"Several literature sources have documented the synthesis of alkenes from α-β-unsaturated carbonyl compounds 11,12 .For instance, this has been achieved through reduction via hydrazone intermediates in the Wolff-Kishner-Huang reduction 13 or through xanthate intermediates in the Barton-McCombie reaction 14 .Cook A et al. reported its accomplishment using a Ni(II) pre-catalyst and a silane reducing agent 15 , while Gómez A M et al. demonstrated the process in two reaction steps 16 .However, the reagents used in these methods are not considered safe or environmentally friendly.To facilitate one-pot cascade reactions, the development of bifunctional catalysts is essential.Various nanostructured materials such as multicompartmentalized mesoporous organosilicas 17 , nanotubes 18 , and hierarchical architectures 19 have been explored to fabricate efficient cascade catalysts.Bifunctional catalysts can be created by loading bimetallic components onto these nanostructured materials, thereby enabling effective catalysis through synergistic interactions between multiple sites."(Please see Pages 1 and 2 in the revised manuscript, marked in blue) 11.Srikrishna, A., Viswajanani, R., Sattigeri, J. A. & Yelamaggad, C. V. Chemoselective reductive deoxygenation of α, β-unsaturated ketones and allyl alcohols.Tetrahedron Lett.36, 2347-2350 (1995).

:Figure 1 .
Figure 1.The structural characterization of ZnNC-X catalysts.c N 2 adsorption-desorption isotherms.(2)The authors did not mention the roles played by carbon which was the most abundant element in their catalysts.Surface functional groups such as -COOH, -C=O and -OH must grow on carbon surfaces which are usually the acidic and basic sites which might play the roles for the hydrogenation of carbonyl to hydroxyl group via the CTH as the authors expected.
Reaction conditions: substrates (0.2 mmol), ZnNC-900 (20 mg), IPA (4 mL), N 2 (1 MPa), poison reagents (20 mg). a 150℃, 11 h; b 180℃, 7 h; c 180 ℃, 24 h.The conversion of substrate and the yield of products were determined by GC with dodecane as internal standard.SupplementaryFigure 7. Effect of reaction time of ZnNC-X catalysts.46.Wang, R. Y. et al.Regulation of the Co-N x active sites of MOF-templated Co@NC catalysts via Au doping for boosting oxidative esterification of alcohols.ACS Catal.12, 14290-14303 (2022).(4)In Fig.2, were there any evidences for the formation of the six-member ring containing D and the L site?Response: Thank you for the important comment.Deuteration experiments are generally used to demonstrate the Meerwein-Ponndorf-Verley (MPV) reduction mechanism, that is, the reaction passes through a six-membered ring mechanism.We are sorry that this issue may not be explained clearly enough in the text.Carbonyl compounds such as aldehydes and ketones are reduced to the corresponding alcohols in isopropyl alcohol with the catalyst, and isopropyl alcohol is oxidized to acetone, which usually involves the MPV reduction mechanism 1 .The MPV reaction mechanism involves the simultaneous coordination of the carbonyl group and the hydroxy group to a metal center, as shown in the following picture 39 .Román-Leshkov Y et al.39  studied the mechanism of glucose isomerization by an intramolecular hydride shift, similar to MPV mechanism, which was proved by deuteration experiment.Gilkey, M. J. and Valekar, A. H. et al.2, 38  also performed deuteration experiments to demonstrate the concerted intermolecular hydride transfer mechanism (MPV).

Table 4 .
Effect of additives on reaction.